Surface contamination analyzer for semiconductor wafers, method used therein and process for fabricating semiconductor device

ABSTRACT

A semiconductor wafer is radiated with an electron beam so that the inelastic scattering takes place in the narrow region, and current flows out from the narrow region; the amount of current is dependent on the substance or substances in the narrow region so that the analyst evaluates the degree of contamination on the basis of the substance or substances specified in the narrow region.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/083,440,filed Feb. 26, 2002 now U.S. Pat. No. 6,753,194, which claims benefit ofpriority under 35 U.S.C. § 119 to Japanese Patent Application No.2001-58075, filed on Mar. 2, 2001.

FIELD OF THE INVENTION

This invention relates to fabrication technologies for semiconductordevices and, more particularly, to a surface contamination analyzer forsemiconductor wafers, a method used therein and a process forfabricating a semiconductor device.

DESCRIPTION OF THE RELATED ART

Semiconductor devices have been enhanced in integration density, and,accordingly, miniature circuit components are integrated on the smallsemiconductor chip. If the surface of a semiconductor wafer iscontaminated with trace elements, the miniature circuit components aremuch liable to be damaged, and the production yield is lowered. Researchand development efforts are being made on 1 giga-bit DRAM (DynamicRandom Access Memory). The contamination due to the trace elements isserious in the process for fabricating the DRAMs. Of course,miniaturization is the key technologies in the development. A cleaningtechnology is also important for the highly reliable ultra large scaleintegration devices. In fact, the requirement for an ultra clean surfaceis getting severe and severe.

Organic compounds in the clean room and plasticizer in the wafercassette are presently seemed to be origins of the contaminants. Theorganic contaminants are adhered to the surfaces of the semiconductorwafers in the form of molecules and/or cluster, and are causative ofreduction in production yield. A particle of organic compound is assumedto be adhered to the surface of a semiconductor wafer. The organiccompound particle is an obstacle in the removal of natural oxide. Eventhough the manufacturer exposes the surface of the semiconductor waferthrough the removal of the natural oxide before the deposition of metal,the organic compound particle does not allow the etchant to remove thenatural oxide therebeneath. This means that a part of the metal layer isheld in contact with the residual natural oxide on the surface of thesemiconductor wafer. Although the resistance in the direct contactbetween the metal layer and the semiconductor wafer is fallen within atarget range, the contact resistance is locally increased, and uniformcontact resistance is not achieved.

Organic contaminant particles are assumed to be adhered to a surface ofsemiconductor wafer. Dopant impurity may be ion implanted into thesurface of the semiconductor wafer/semiconductor layer. The organiccontaminant particles are also obstacle against the ion-implantation,and do not allow the impurity region to have a target impurity profile.If the impurity region is used for a channel region or diode, the fieldeffect transistor or diode does not exhibit designed characteristics.Especially, the channel region forms a part of flash memory. Electronsor holes are injected and evacuated through the gate oxide layer betweenthe channel region and the floating electrode. The residual organiccontaminant particles accelerate the aged deterioration, and,accordingly, reduce the duration of life as reported by ToshiyukiIwamoto in “Research for Highly Reliable Extremely Thin Oxide Layer”,dissertation for a doctor degree, Tohoku University, March, 1998.

Semiconductor wafers are concurrently conveyed from an apparatus toanother as a lot. Since the time period consumed in an apparatus isdifferent from the time period consumed in another apparatus, thesemiconductor wafers are to wait until the previous lot is unloaded fromthe apparatus, and are exposed to the atmosphere in the clean room.While the semiconductor wafers are being exposed to the atmosphere, themolecular contaminants or contaminant clusters are unavoidably adheredto the surfaces of the semiconductor wafers. The waiting time isdifferent between the lots, and, accordingly, the semiconductor wafersare different in degree of contamination from one another. Thecontaminants are influential in the electric properties of thesemiconductor wafers. Thus, accurate evaluation technologies arerequired for the ultra large-scale integration devices.

A wide variety of material is used in the processes for fabricating theultra large-scale integration devices. Silicon dioxide, i.e., SiO₂ ispopular to the semiconductor device manufacturers. Other kinds ofinsulating material such as, for example, SiOF, HSQ, SiN and Ta₂O₅ areemployed in the ultra large-scale integration devices. Semiconductorwafers are cleaned in certain cleaning solution such as water solutioncontaining sulfuric acid and hydrogen peroxide or water solutioncontaining ammonia and hydrogen peroxide. The contaminants are removedthrough chemical reactions with these kinds of cleaning solution.However, new cleaning technologies are required for the new kinds ofmaterial. Physical phenomena are employed in the cleaning technologiesfor those kinds of material. The contaminants are, by way of example,removed from semiconductor wafers by using mega-sonic or ultra sonic.These cleaning technologies are effective against contaminants on thesurfaces of the semiconductor wafers. However, the cleaning solutionand/or the physical energy hardly reaches the contaminants in deepvalleys or trenches and micro-holes formed in the three-dimensionalstructure. If the contaminants are left on the bottom surfaces of thetrenches and micro-holes, the residual contaminants vary the electricproperties, and the circuit components formed in the trenches andmicro-holes do not exhibit designed characteristics. The defectivecircuit components are causative of reduction in production yield. Eventhough the products pass the inspections, malfunction is liable to takeplace in the products, and, accordingly, those products are lessreliable.

In order to cope with the contamination, it is necessary to evaluate thesurface contamination without any damage to the complicated surfaceconfiguration for optimizing the cleanings. A total reflectionfluorescent x-ray spectroscopy has been developed for the surfaceevaluation. The total reflection fluorescent x-ray spectroscopy isusually abbreviated as “TXRF”. Monochromatic x-ray is radiated to thesurface of a semiconductor wafer at a small incident angle. Thesemiconductor wafer and contaminants generate fluorescent x-ray to asemiconductor detecting unit, and the contaminants are specified on thebasis of the fluorescent x-ray incident onto the semiconductor detectingunit. Metal contaminants are well detected through the total reflectionfluorescent x-ray spectroscopy. However, contaminants essentiallycomposed of carbon-containing/nitrogen-containing molecules are hardlydetected through the total reflection fluorescent x-ray spectroscopy.Moreover, the mono-chromatic x-ray does not reach the bottom surfaces ofextremely deep trenches, because the incident angle is small, and thecontaminants on the bottom surfaces are not detectable.

A surface contamination analyzer with an optical detector is disclosedin Japanese Patent Application laid-open No. 7-221148. Infrared light isradiated from a light source to the surface of a semiconductor wafer,and the reflected infrared light is incident on the surface at apredetermined angle. The infrared light is reflected on the surface,again, and the reflection is analyzed through a spectral analysis. Thecontaminants essentially composed ofcarbon-containing/nitrogen-containing molecules are detected withoutserious damage to the semiconductor wafer.

Another prior art surface contamination analyzer is disclosed inJapanese Patent Application laid-open No. 9-243535. Carrier gas flowsover the surface of a semiconductor wafer, and laser light is radiatedonto a certain area of the surface. The contaminants are vaporized inthe radiation of the laser light, and the gaseous contaminant is ionizedtogether with the carrier gas. The ionized gas is analyzed through amass spectrometry, and the contaminants are specified on the basis ofthe analysis result. Even if the contaminants are on the bottom surfacesof the extremely deep trenches, the laser light reaches the contaminantsso as to vaporize them. Thus, the contaminants in the extremely deeptrenches are analyzable through the second prior art surfacecontamination analysis.

A problem inherent is encountered in the first prior art surfacecontamination analysis technology in a small signal-to-noise ratio ofthe reflection from the trenches/holes with large aspect ratios. This isbecause of the fact that the first prior art surface contaminationanalyzer is to keep the incident light at the predetermined angle. Inother words, the limit is set on the incident angle in the surfacecontamination analysis so that the reflected infrared light does notreach the bottoms of the trenches/holes with the large aspect ratios.

On the other hand, a problem inherent in the second prior art surfacecontamination analyzer is a small signal-to-noise ratio of the vaporgenerated from an extremely narrow area. When the vapor is generatedfrom a relatively wide area, a large amount of contaminants isvaporized, and the mass spectrometry is surely reliable. However, theamount of contaminants vaporized from an extremely narrow area is toolittle to analyze it through the mass spectrometry. Thus, the secondprior art surface contamination analyzer is not available for a detailedreport.

If the analysis on the surface contamination is inaccurate, it isdifficult to optimize the cleaning against the contamination, and theresidual contaminants are carried from an apparatus to another throughthe contaminated semiconductor wafer. This results in reduction inproduction yield and poor reliability of semiconductor devices. Thesurface contamination analysis is an important step in the process forfabricating a semiconductor device.

SUMMARY OF THE INVENTION

It is therefore an important object of the present invention to providea surface contaminant analyzer, which is available for a detailed reporton a surface contamination with carbon, nitrogen-containing molecules orcluster regardless of the surface configuration of a specimen.

It is also an important object of the present invention to provide ananalyzing method used in the surface contamination analyzer.

It is another important object of the present invention to provide aprocess for fabricating a semiconductor device through which theproduction yield and reliability of semiconductor device are enhanced.

In accordance with one aspect of the present invention, there isprovided a surface contamination analyzer comprising an electron beamradiating unit including an electron gun for radiating an electron beamalong a certain path, and a current measuring unit including a walldefining a chamber into which the certain path extends, a movable stagemounting a sample and moved so as to align a target region with thecertain path and a current measuring equipment electrically connectedbetween the sample and a constant voltage source for measuring theamount of current flowing out from the target region under the radiationof the electron beam onto the target region.

In accordance with another aspect of the present invention, there isprovided a method for investigating a degree of contamination on atarget region of a contaminated sample comprising the steps of a)aligning the target region of the contaminated sample with a path of anelectron beam, b) measuring the amount of current flowing out from thetarget region under radiation of the electron beam, c) comparing theamount of current with the amount of reference current flowing out froma region of a reference sample corresponding to the contaminated samplefor determining a difference between the amount of current measured andthe amount of reference current, and d) determining the degree ofcontamination on the target region on the basis of the difference.

In accordance with yet another aspect of the present invention, there isprovided a process for fabricating a semiconductor device comprising thesteps of a) treating the semiconductor water in an atmospherepotentially having an origin of contamination, b) investigating a degreeof contamination on the semiconductor wafer through sub-steps of b-1)aligning a target region of the semiconductor wafer with a path of anelectron beam, b-2) measuring the amount of current flowing out from thetarget region under radiation of the electron beam, b-3) comparing theamount of current with the amount of reference current flowing out froma region of a reference wafer corresponding to the semiconductor waferfor determining a difference between the amount of current measured andthe amount of reference current and b-4) determining the degree ofcontamination on the semiconductor wafer on the basis of the difference,c) evaluating the degree of contamination to see whether or not acleaning is required for the semiconductor wafer, d) decontaminating thesemiconductor wafer when the answer at step c) is given affirmative ande) treating the semiconductor wafer in another next atmosphere.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the surface contamination analyzer,method and the process for fabricating a semiconductor device will bemore clearly understood from the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic view showing the arrangement of a surfacecontamination analyzer according to the present invention;

FIG. 2 is a cross sectional view showing the structure of a siliconwafer used in an experiment;

FIG. 3 is a graph showing the results of a time-flight secondary ionmass spectrometry;

FIG. 4 is a graph showing a dispersion of contamination on a siliconwafer;

FIG. 5 is a schematic view showing the arrangement of another surfacecontamination analyzer according to the present invention;

FIG. 6 is a graph showing the relation between the incident angle of anelectron beam and the amount of current measured;

FIG. 7 is a graph showing the relation between the acceleration energyof an electron beam and the projection range of electrons;

FIG. 8 is a flowchart showing a process sequence containing the surfacecontamination analysis according to the present invention; and

FIG. 9 is a flowchart showing another process sequence containing thesurface contamination analysis according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, an electron beam is radiated onto atarget substrate and a reference substrate the surface state of whichhas been already known. The electron beam gives rise to current flowingthrough the target substrate and the reference substrate to a constantvoltage source. The amount of current flowing through the targetsubstrate is compared with the amount of current flowing through thereference substrate, and the surface contamination of the targetsubstrate is evaluated on the basis of the difference therebetween.

Assuming now an narrow target area is contaminated with chemicalmolecules or cluster, the electron beam radiation gives rise tocontinuous inelastic scattering in the chemical molecules/cluster aswell as the substrate. The primary electron looses part of the energy,and part of the excited electron does not reach the surface of thesubstrate. The atoms are partially ionized. For this reason, theelectric charges do not balance in the bulk, i.e., the chemicalmolecules/cluster and the substance forming the substrate. In order torecover the balance, the current flows through the substrate. Thecurrent is called as “absorption current”. The amount of current isstrongly dependent on the electron scattering in the material formingthe substrate. This is because of the fact that the band gap isdifferent from each other. Semiconductors have band gaps wider thanthose of metals and narrower than those of insulators. In metal, thesecondary electrons are much liable to exceed the forbidden band, andlose the energy through the interaction with the electrons in thelattice. On the other hand, the interaction hardly takes place in theinsulator, because the forbidden band is wide. In other words, theelectrons are less liable to lose the energy loss. Thus, the amount ofcurrent is varied depending upon the material radiated with the electronbeam. Accordingly, the material is specified by comparing the amount ofcurrent measured with those of reference substrate. It is preferable toradiate the electron beam for a long time. The current is generated forthe long time, and the amount of current is accumulated in the sameperiod. The difference becomes large, and the substance in thetarget-region becomes clearly discriminative.

As will be understood, the electron beam is radiated onto the narrowarea on the substrate at a predetermined acceleration energy. Theelectron beam causes the electron scattering to take place in thesurface portion of the order of tens nanometers thick, which is equal tothe mean range of the electron beam. If a piece of hydrocarbon compound,which is electrically insulating, is adhered to the surface of thesubstrate, the absorption current is different in amount from thatflowing through the substrate without any hydrocarbon compound. Thesurface state of the substrate is evaluated on the basis of thedifference in the amount of absorption current.

There is not any limit on the incident angle of the electron beam. Evenif a contact hole with a large aspect ratio is formed in the surfaceportion of the substrate, the electron beam teaches the bottom of thecontact hole by adjusting the incident angle to a large value. Theabsorption current flows out through the reverse surface of thesubstrate. The signal-to-noise ratio is large enough to analyze thematerial exactly.

In another embodiment, the electron beam is radiated at an angle so asto measure the amount of current, and electron beam radiation isrepeated at different angles so as to measure the amount of current ineach electron beam radiation. In this instance, the amount of currentflowing through the target substrate at each angle is compared with theamount of current flowing through the reference substrate at the sameangle. The surface contamination is accurately evaluated through themultiple comparison.

In yet another embodiment, the target substrate is treated with heat ininert gas prior to the radiation with the electron beam, and the surfacecontamination is evaluated as described hereinbefore. The heat treatmentmay be repeated at different temperatures so as to measure the amount ofcurrent after each heat treatment. The accuracy of the evaluation isenhanced.

In still another embodiment, the electron beam is accelerated at acertain value of the acceleration energy, and the electron beamradiation is repeated at different values of the acceleration energy.The amount of current is measured at each electron beam radiation, andis compared with that measured through the reference substrate.

According to the present invention, surface contamination analyzers eachcomprise current measuring units and electron beam radiating units. Thecurrent measuring unit has a movable stage in a chamber thereof and anampere meter, and the target substrate and the reference substrate areplaced on the movable stage. The relative position between thetarget/reference substrates and the electron beam radiating unit isvaried by means of the movable stage. The target substrate and thereference substrate are electrically connected in parallel to theconstant voltage source such as, for example, the ground. The electronbeam radiating unit radiates an electron beam onto the target substrateand the reference substrate placed on the movable stage. The electronbeam is focused on a narrow target area on the surface of thetarget/reference substrate. Then, current flows to the constant voltagesource, and the amount of current is measured by means of the amperemeter.

In a surface contamination analyzer implementing one of the embodiments,the movable stage is inclined with respect to the virtual surface wherethe target/reference substrates are moved relatively to the electronbeam. The radiation angle is varied by inclining the target/referencesubstrates.

The surface contamination analysis forms at least one step in a processfor fabricating semiconductor devices. Various treatments such as, forexample, diffusion, oxidation, deposition and ion-implantation areselectively carried out on a semiconductor wafer at different steps ofthe fabrication process. When the semiconductor wafer reaches the stepof evaluating the surface state of the semiconductor wafer, thesemiconductor wafer is placed on the movable stage as the targetsubstrate. The surface contamination is investigated as describedhereinbefore, and the result is evaluated to see whether or not acleaning step is required for the steps to be carried out. If thesurface contamination is serious, the semiconductor wafer is cleanedbefore proceeding to the next step. On the other hand, when the surfacecontamination is negligible, the semiconductor wafer proceeds to thenext step. Plural sorts of cleaning step may be prepared for thesemiconductor wafer, and is selectively carried out depending upon theevaluation result.

First Embodiment

Referring to FIG. 1 of the drawings, a surface contamination analyzerembodying the present invention largely comprises a current measuringunit 1 and an electron beam radiating unit 2. The current measuring unit1 is formed with a chamber, and the electron beam radiating unit 2projects into the chamber. A target such as, for example, asemiconductor wafer 101 is placed in the current measuring unit 1, andis grounded. The electron beam 102 is radiated from the electron beamradiating unit 2 onto the semiconductor wafer 101. The semiconductorwafer 101 is on the way to the end of a process for fabricatingsemiconductor devices, and a major surface 101 a of the semiconductorwafer 101 is rough due to trenches, contact holes and lower-level wiringlayers. The electron beam 102 is so thin that bottom surfaces of thevalleys are radiated with the thin electron beam 102.

The current measuring unit 1 is formed with a chamber 1 a, and a movablestage 103 is provided in the chamber 1 a. The movable stage 103 ismovable on a virtual plane perpendicular to the electron beam 102. Anarrow area to be radiated with the electron beam is specified in anorthogonal coordinate system or a polar coordinate system. The stage 103is movable in the two directions perpendicular to each other or in bothof the direction of radius vector and the angle. The semiconductor wafer101 a is placed on the movable stage 103, and the movable stage 103aligns the target region with the electron beam 102.

The current measuring unit 1 further comprises a collecting electrode108 and an ampere meter 109. The collecting electrode 108 is insertedbetween the movable stage 103 and the semiconductor wafer 101, and isconnected through the ampere meter 109 to the ground 110. The connectionbetween the collecting electrode 108 and the ampere meter 109 is onlyone current path so that the absorption current does not flow into theother component such as the movable stage 103.

The electron beam radiating unit 2 comprises an electron gun 104, ananode 105, a constant voltage source 106 and an electron lens 107. Theelectron gun 104 is powered by the constant voltage source 106, andradiates the electron beam 102 toward the movable stage 103. The anode105 is appropriately biased by the constant voltage source 106, andaccelerates the electron beam 102. The electron lens 107 is providedbetween the electron gun 104 and the semiconductor wafer 101 on themovable stage 103, and controls the amount of current of the electronbeam 102 and the convergence of the electron beam 102.

When the surface contamination in a target region of the semiconductorwafer 101 is to be investigated, the semiconductor wafer 101 is placedon the collecting electrode 108, and the movable stage 103 makes thetarget region aligned with path through which the electron beam 102 isradiated. The electron beam 102 is radiated from the electron gun 104,and is accelerated in the electric field created by the anode 105. Theelectron beam 102 is converged by the electron lens 107, and is incidentonto the target region on the major surface 101 a. The electronscattering takes place in the target region, and the current flows intothe collecting electrode 108. The current flows from the collectingelectrode 108 through the ampere meter 109 into the ground 110. Theelectron beam radiation is continued for a certain time period, and thevalue of current is accumulated in the ampere meter 109. The accumulatedvalue is unique to a contaminant so that the analyst specifies thecontaminant on the basis of the accumulated value

The present inventors carried out the following experiment, andconfirmed that the surface contamination was accurately evaluatedthrough the surface contamination analysis according to the presentinvention.

First, the inventors prepared two silicon wafers. The diameter of thesilicon wafers was 200 millimeters. The silicon wafers werep-conductivity type, and the resistivity was of the order of 1 ohm-cm.

Silicon oxide was deposited over the entire surfaces of the siliconwafers by using a chemical vapor deposition, and the silicon oxidelayers were 1000 nanometers thick. Photo-resist masks were formed on thesilicon oxide layers through the photo-lithography, and the siliconoxide layers were selectively etched by using a dry etching technique.Recesses penetrated through the silicon oxide layers, and reached themajor surfaces of the silicon wafers as shown in FIG. 2. The diameter ofthe recesses ranged from 0.25 microns to 3 microns. The recesses were1000 nanometers deep. In FIG. 2, reference numerals 101 and 111designate the silicon wafer and the silicon oxide layer, respectively.One of the recesses is labeled with reference numeral 112, and thebottom surface is designated by reference numeral 113. The photo-resistmasks were ashed, and the silicon wafers 101 were cleaned through aseries of cleaning/rinse techniques using the water solution containingsulfuric acid and hydrogen peroxide, water solution containing ammoniaand hydrogen peroxide and pure water. Organic compound and alkalinemetals were eliminated from the resultant structure shown in FIG. 2.

One of the silicon wafers was placed in a closed space where syntheticoil was vaporized. The silicon wafer was contaminated with the syntheticoil, i.e., hydrocarbon compounds. The other silicon wafer was stored ina clean atmosphere. The inventors used the silicon wafer contaminatedwith the hydrocarbon and the clean silicon wafer as the target substrateand the reference substrate, respectively.

The inventors further prepared a pair of silicon wafers. The siliconwafers were deposited with the silicon oxide, and the recesses wereformed in the silicon oxide layers of 1000 nanometers thick. One of thesilicon wafers was contaminated with the vapor of the synthetic oil, andthe other was stored in the clean atmosphere. Thus, one of the siliconwafer was contaminated with the hydrocarbon compound as similar to thetarget substrate, and the other silicon wafer was kept clean as similarto the reference substrate. Thus, two pairs of silicon wafers wereobtained.

The inventors firstly analyzed one of the two pairs of silicon wafers byusing a time-flight secondary ion mass spectrometer. The contaminatedsilicon wafer and the clean silicon wafer were radiated with an ionbeam. Then, the contaminants were scattered through the sputtering so asto analyze the contaminants directly. However, the surfaces of thesilicon wafers were damaged due to the ion-bombardment. The ionicspecies were Ga⁺, the accelerating energy was adjusted to 25 KeV. Ioniccurrent was 1.2 pA, and the incident angle was 45 degrees. The massspectrometry was carried out in the bunching-repetition mode at 10 KHz.An anti-static correction was not carried out. The analyzed region was100 micron square. The results of the time-flight secondary ion massspectrometry was shown in FIG. 3. Black columns and white columns stoodfor the relative strength on the contaminated silicon wafer and therelative strength on the clean silicon wafer, respectively.

Subsequently, the inventors placed the contaminated silicon wafer andclean silicon wafer of the other pair on the collecting electrode 108,and the target regions were successively aligned with the path for theelectron beam 102 by controlling the movable stage 103. The electronbeam 102 was radiated from the electron gun 104, and was accelerated inthe electric field created by the anode 105. The acceleration energy wasadjusted to 500 eV, and the beam current was 50 pA. The electron beam102 was focused on the target region through the electron lens 107.Then, the electron beam 102 gave rise to the electron scatteringphenomenon, and the electric charges lost the balance in the targetregion. In order to recover the balance, the current flew through theampere meter 109, and the amount of current was accumulated for 200millisecond.

The electron beam 102 accelerated at 500 eV gave rise to the electronscattering phenomenon in the surface portion of the silicon wafers ofthe order of several nanometers. However, the acceleration energy mightbe adjusted to a value between 1 eV and 500 eV depending upon thecontaminated substrate. However, when clusters of tens nanometers tohundreds nanometers were predictable on the target region, theacceleration energy was adjusted to a value between 500 eV and 10000 eV.The beam current at 50 pA prevented the silicon wafers from charge-upphenomenon. The beam current between 1 pA and 50 pA was preferable fromthe view point against the charge-up phenomenon. However, if the beamcurrent was increased between 50 pA and 1 nA, the signal-to-noise ratiowas improved. The present inventors confirmed that the surfacecontamination was analyzed with the electron beam 102 at 1 eV-10000 eVat 1 pA-1 nA.

The electron beam 102 was regulable in the wide beam current range,i.e., 1 pA to 1 nA, and the lower limit, i.e., 1 pA was so small thatthe electron beam radiating unit 2 could make the electron beam 102extremely narrow. The analyst precisely investigated the surfacecontamination over the target substrate with the extremely narrowelectron beam 102 at a large signal-to-noise ratio. The presentinventors confirmed that the contaminants in the bottom of a micro-holeless than 0.25 micron in diameter were analyzed by using the surfacecontamination analyzer according to the present invention.

The upper limit of the acceleration energy was so large that theelectron beam 102 could penetrate a thick silicon oxide layer, dopedsilicon oxide layer, silicon nitride layer and organic compound layers.The present inventors confirmed that the electron beam penetratedthrough a silicon oxide layer greater in thickness than 1000 nanometers.

The inventors investigated various substrates. The inventors depositedmetal, i.e., aluminum and titanium or semiconductor, i.e.,silicon-germanium and gallium arsenide over the silicon wafers, andfurther deposited silicon oxide over the metal/semiconductor layer.Micro-holes were formed in the silicon oxide layers, and the substrateswere contaminated with the hydrocarbon compound. The inventors evaluatedthe surface contamination, and confirmed that the surface contaminationwas accurately evaluated by using the surface contamination analyzingtechnique described hereinbefore. This is because of the fact that theband gaps in the hydrocarbon compounds are different from those of themetal and those of the other kinds of semiconductor.

The inventors measured the accumulated current value over the majorsurfaces of the silicon wafers 101. The contaminants were dispersed overthe major surface of the contaminated silicon wafer 101 as shown in FIG.4. The central area and the corners was heavily contaminated, and thecontamination became light toward the peripheral area. However, theperipheral area was locally contaminated. On the other hand, thedispersion was not observed in the clean silicon wafer. Accordingly, theaccumulated current value was widely varied on the major surface of thecontaminated silicon wafer, and the average current value was 1.58 pA.On the other hand, the accumulated current value was almost constantover the major surface of the clean silicon wafer, and the averagecurrent value was 2.03 pA. Thus, the contaminated silicon wafer wassmaller in average current value than the clean silicon wafer. Theinventors concluded that it was possible to evaluate the surfacecontamination with hydrocarbon on semiconductor wafers by comparing theaverage current value with that of the clean silicon wafer, i.e., thereference substrate without any damage to the silicon wafers.

If the contaminated region have been known, the analyst directly alignsthe contaminated region with the electron beam path. However, if thecontaminated region is unknown, the analyst stepwise aligns narrowregions with the electron beam path, and repeats the electron beamradiation and measurement of the current at each point. When the entiresurface of the target substrate was radiated with the electron beam, theanalyst can obtain a dispersion of the contamination over the entiresurface on the basis of variation of measured current value.

As will be appreciated from the foregoing description, the electron beamis radiated onto a narrow region on the major surface of the targetsubstrate so that the inelastic scattering takes place in the radiatedregion. The scattering phenomenon gives rise to current, and the amountof current is dependent on the substance in the radiated region, becauseconductive material, semiconductor material and insulating material haveeach band gaps different from one another. The surface contaminationanalysis according to the present invention is carried out on the basisof the difference in current. The amount of current flowing out from thetarget substrate is measured for a certain time period, and thesubstance is specified on the basis of the accumulated current value.

There is not any limit on the incident angle of the electron beam. Thismeans that, even if a deep hole with a large aspect ratio is formed inthe target region, the electron beam can reach the bottom of the deephole. Thus, the surface contamination in the deep hole is analyzablethrough the surface contamination analyzing technique according to thepresent invention at a large signal-to-noise ratio.

Moreover, the electron beam is focused on an extremely narrow region inso far as the electron beam current is fallen within the wide range,i.e., 1 pA to 1 nA. This means that the target substrate is preciselyanalyzed by using the surface contamination analyzing technique at alarge signal-to-noise ratio. Thus, the surface contamination analysisaccording to the present invention is advantageous over the prior artsurface contamination analyzing techniques.

Second Embodiment

Turning to FIG. 5 of the drawings, another surface contaminationanalyzer embodying the present invention also comprises a currentmeasuring unit 1A and the electron beam radiating unit 2, which issimilar in structure to the electron beam radiating unit incorporated inthe first embodiment. Component parts of the electron beam radiatingunit 2 are labeled with references designating the correspondingcomponent parts of the first embodiment without detailed description.

The current measuring unit 1A is similar in structure to the currentmeasuring unit 1 except a movable stage 103A. Although the movable stage103 is two-dimensionally moved on a virtual plane 103 perpendicular tothe electron beam 102, the movable stage 103A is inclined from thevirtual plane 103B as well as the two-dimensional motion on the virtualplane 103B. In other words, the movable stage 103A varies the angle γbetween the virtual plane 103B and the major surface of the targetsubstrate 101.

The surface contamination analysis basically advances as similar to thefirst embodiment. A difference from the surface contamination analysisimplementing the first embodiment is that the electron beam radiation isrepeated by changing the incident angle.

The surface contamination analysis starts with preparation of a cleansubstrate and a contaminated substrate. Each of the clean/contaminatedsubstrates is placed on the collecting electrode 108 on the movablestage 103A, and the current flowing into the ground 110 is measured forspecifying the substance of the target region. If the target region hasbeen already known, the movable stage 103A directly aligns the targetregion with the path of the electron beam 102, and the electron beamradiating unit 2 radiates the electron beam onto the target region. Thecurrent flows through the ampere meter 109, and the amount of current ismeasured for a certain time period. The movable stage 103A changes theangle γ, and the target region is radiated with the electron beam 102 atdifferent incident angle. The current is measured for the certain timeperiod. The electron beam radiation and the measurement are repeated atdifferent incident angles.

The inventors prepared the silicon wafers 101 formed with the recesses112, and the recess 112 was aligned with the path of the electron beam102 (see FIG. 2). The inventors adjusted the acceleration energy and theelectron beam current to 500 eV and 50 pA, respectively, and the currentwas measured for 200 millisecond. The incident angle was stepwise variedfrom −π radian to π radian, and the amount of current measured atdifferent incident angles was plotted as shown in FIG. 6. The amount ofcurrent was maximized at 0 radian., i.e., normal to the major surface,because the electron beam 102 reaches the bottom 113 of the recess 112.The amount of current was reduced together with the incident angle.Thus, the amount of current exhibited angle dependency.

The profile representative of the angle dependency was available for thethree-dimensional contamination analysis. When the silicon wafercontaminated with the vapor of synthetic oil was radiated with theelectron beam 102 at different incident angle, the profilerepresentative of the angle dependency was different from the profileshown in FIG. 6 depending upon the surface contaminated with thehydrocarbon compounds. The inventors investigated the surfacecontamination after the dry etching on the silicon oxide layer 111. Theresidual hydrogen fluoride and hydrocarbon compound were left on thebottom surface of the recess 112. In the surface contamination analysis,the electron beam radiated at 0 radian reached the bottom 113 of therecess 112, and the contaminants was influential in the amount ofcurrent. However, the electron beam 102 obliquely radiated passed overthe contaminants, and the amount of current was free from thecontaminants. Thus, the profile was locally deformed, and the locallydeformed profile exhibited the three-dimensional dispersion of thecontaminants. The inventors concluded that the contamination wasthree-dimensionally analyzable by using the surface contaminationanalyzer implementing the second embodiment.

Third Embodiment

The surface contamination analysis implementing the third embodimentcomprises the step of treating a semiconductor wafer or target substratewith heat in inert atmosphere at a predetermined temperature and thestep for measuring the current flowing out from the semiconductor waferunder the electron beam radiation. These steps may be repeated. Thesurface contamination analyzer shown in FIG. 1 or 5 is used in thesecond step.

The inventors prepared a contaminated silicon wafer. The contaminatedsilicon wafers was placed on the movable stage 103A, and the targetregion was aligned with the path of the electron beam 102. Theacceleration energy was 500 eV, and the electron beam current was 50 pA.The electron beam 102 was radiated from the electron gun 104 through theelectron lens 107 onto the target region. The electron beam 102 gaverise to the electron scattering in the target region, and the currentflew out from the target region. The amount of current was measured, andthe accumulation time was 200 millisecond. The current was 1.61 pA.

Subsequently, the contaminated silicon wafer was placed in nitrogenatmosphere created in a chamber of a furnace. The nitrogen atmospherewas 70 degrees in centigrade, and the contaminated silicon wafer wastreated with heat for 1 minute. The inventors repeated the measurement.The amount of current was 1.76 pA.

The contaminated silicon wafer was placed in the nitrogen atmosphere at70 degrees in centigrade, again, and the temperature was increased to200 degrees in centigrade at 30 degrees in centigrade per minute. Thecontaminated silicon wafer was kept in the high temperature nitrogenatmosphere for 1 minute. The inventors took out the contaminated siliconwafer from the furnace, and measured the current under the electron beamradiation. The current was increased to 1.89 pA.

The inventors put the contaminated silicon wafer into the hightemperature nitrogen atmosphere at 200 degrees in centigrade, and raisedthe temperature at 10 degrees in centigrade per minute. When thenitrogen atmosphere reached 300 degrees in centigrade, the inventorsstopped the temperature rise, and kept the contaminated silicon wafer inthe high temperature nitrogen atmosphere at 300 degrees in centigradefor one minute. The inventors took out the contaminated silicon waferfrom the chamber, and measured the current under the electron beamradiation. The current was increased to 2.07 pA.

The increase of the current was derived from the evaporation of certaincontaminants. Some contaminants were evaporated at 70 degrees incentigrade, and several contaminants were evaporated at 200 degrees incentigrade. Other contaminants were evaporated at 300 degrees incentigrade. Thus, the contaminated silicon wafer was stepwise cleaned,and the amount of current was increased. The difference in measuredcurrent was unique to the contaminants removed from the contaminatedsilicon wafer. Thus, the contaminants were specified on the basis of thedifferent in current measured after the heat treatment at a certaintemperature.

The stepwise evaporation may be available for the surface contaminationanalysis from another point of view. An analyst is assumed to specify acontaminant. The analyst places the contaminated substrate in inertatmosphere, and heats the contaminated silicon wafer to the evaporationtemperature of the known contaminant. The known contaminant is removedfrom the contaminated substrate, and the next analysis becomes simple.

Fourth Embodiment

In yet another surface contamination analysis embodying the presentinvention, the electron beam radiation is repeated under differentconditions. The surface contamination analyzer shown in FIG. 1 or 5 isavailable for the surface contamination analysis implementing the fourthembodiment.

The inventors prepared a contaminated silicon wafer, and placed it onthe collecting electrode 108. A target region was aligned with the pathof the electron beam 102, and the electron beam 102 was radiated ontothe target region at 500 eV at 50 pA. The current flew out from thetarget region, and the inventors measured the amount of current. Thecurrent was 1.60 pA.

Subsequently, the inventors adjusted the acceleration energy to 1000 eV,and the measured the current under the electron beam radiation at 50 pA.The amount of current was 2.24 pA. The inventors further increased theacceleration energy to 1500 eV, and measured the current under theelectron beam radiation at 50 pA. The amount of current was increased to3.48 pA. Thus, the amount of current was varied together with theacceleration energy.

FIG. 7 shows a relation between the acceleration energy and theprojection range of implanted electrons. The stronger the accelerationenergy, the longer the projection range. The implanted electron beamcauses the inelastic scattering to take place, and the inelasticscattering gives rise to the generation of the current flowing out fromthe region therearound. When the electron beam 102 reaches the deepregion, the electrons are generated through the scattering more thanthose generated by the electron beam terminated at a shallow region.When a large organic compound cluster of tens nanometers to hundredsnanometers exists in the target region, the electron beam 102 isterminated at a certain region in the large cluster in so far as theacceleration energy is weak. However, the electron beam reaches a deeperportion together with the increase of acceleration energy, and finally,reaches the silicon wafer. Accordingly, the amount of current wascontinuously increased in the large organic compound cluster, and isdrastically varied at the boundary between the large organic cluster andthe silicon wafer. The analyst can determine the size of the largeorganic compound cluster on the basis of the acceleration energy at thedrastic change.

Contribution to Fabrication Process

If a contaminated semiconductor wafer is conveyed from one apparatus toanother, the contaminants are also carried to the next stage, and thecontamination is spread over the fabrication system. In order to preventthe fabrication system from the contamination, it is necessary toevaluate the surface contamination of silicon wafers. If thecontamination is serious, the silicon wafer is to be cleaned.

FIG. 8 shows a process for fabricating semiconductor devices, and thesurface contamination analysis according to the present invention isincorporated in the process. The process may be categorized in a singlewafer processing.

A cleaning, drying, etching, photo-resist removal, ion-implantation orashing is, by way of example, carried out at step S11. The cleaningmeans a wet cleaning, which may be carried out in water solutioncontaining sulfuric acid and hydrogen peroxide which is abbreviated as“SPM”, water solution containing ammonia and hydrogen peroxide which isabbreviated “APM or SC-1”, diluted hydrofluoric acid which isabbreviated as “DHF”, brush-scrubbing, hydrogen-containing water andozone-containing water. The wet cleaning may be assisted with megasonicor ultrasonic. When the previous stage is completed, the manufacturermakes a decision whether or not the semiconductor wafer is to beinvestigated for the surface contamination as by step S12. If surfacecontamination is predicted, the answer is given affirmative, and thesurface contamination analysis is carried out as by step S13. On theother hand, there is not any serious contamination source in theprevious stages. The answer at step S12 is given negative, and themanufacturer decides the process to proceed to the next stage at stepS16. A heat treatment, thin film growth, deposition, ion-implantation,dry etching or photolithography is carried out in the next stage.

The manufacture is assumed to decide the semiconductor wafer to beinvestigated at step S12. The semiconductor wafer is subjected to thesurface contamination analysis. The semiconductor wafer is put on themovable stage 103/103A, and a target region is aligned with the path ofthe electron beam 102. The electron beam 102 is radiated onto the targetregion at a predetermined acceleration energy and at a predeterminedbeam current. The scatterine takes place, and gives rise to generationof current flowing out from the target region. The current value isaccumulated for a predetermined time period. The semiconductor wafer maybe inclined so as to repeat the electron beam radiation at differentangles. The movable stage 103/103A makes the next target region alignedwith the path of the electron beam 102, and the electron beam radiationis repeated so as to measure the current. When the current is measuredin all of the target regions or entire surface of the semiconductorwafer, the analyst proceeds to step S14.

The analyst checks the accumulated current values to see whether or notthe degree of contamination exceeds a critical value as by step S14.First, the accumulated current values are subtracted from accumulatedcurrent values measured in a reference semiconductor wafer, and thedifferences are absolutized. The absolute values are representative ofthe degree of contamination. The absolute values are compared with amargin to be allowed in the next stage, i.e., the critical value. Themargin has been experimentally determined. If the absolute values areless than the margin, the answer at step S14 is given negative, and themanufacturer proceeds to the next stage. On the other hand, when theabsolute values are equal to or greater than the margin, the surfacecontamination is serious, and the answer at step S14 is givenaffirmative.

The manufacturer proceeds to step S14, and the semiconductor wafer iscleaned through an appropriate cleaning technique so as to remove thecontamination from the semiconductor wafer.

EXAMPLES

The inventors investigated the effects of the fabrication processcontaining the surface contamination analysis. The inventors preparedtwo 8-inch silicon wafers. Silicon oxide was thermally grown to 10nanometers thick on the major surfaces of the silicon wafers, and,thereafter, the two silicon wafers were left in the clean room for aweek. One of the silicon wafers, i.e. the first silicon wafer wasassumed that the negative answer was given at step S12, and the othersilicon wafer, i.e., the second silicon wafer was assumed that thepositive answer was given at step S12.

The second silicon wafer was investigated at step S13. The surfacecontamination was serious, and the answer at step S14 was givenaffirmative. With the positive answer at step S14, the second siliconwafer was cleaned through a series of cleaning steps using watersolution containing sulfuric acid and hydrogen peroxide, water solutioncontaining ammonia and hydrogen peroxide and diluted hydrofluoric acid,and, thereafter, the second silicon wafer was rinsed in pure water.

The inventors evaluated the first silicon wafer and the second siliconwafer by using FT-IR method. The contaminants on the second siliconwafer were CH₂ at 0.5×10¹⁴ molecules/cm² and CH₃ at 0.13×10¹⁴molecules/cm² after step S15. On the other hand, the first silicon waferwas not subjected to neither surface contamination analysis at step S13nor cleaning at step S15, and the contaminants on the first siliconwafer were 10×10¹⁴ molecules/cm² and CH₃ at 1.1×10¹⁴ molecules/cm². Thepresent inventors confirmed that the surface contamination analysisfollowed by the cleaning was effective against the spread of thecontaminants over the fabrication system. Only the seriouslycontaminated silicon wafers were cleaned so that the surfacecontamination analysis according to the present invention is economical.

The inventors selectively carried out the cleaning by using themegasonic, the cleaning in the ozone-containing wafer, the cleaning inthe hydrogen-containing water and the brush-scrub cleaning at step S115,and confirmed that the silicon wafers were decontaminated. The inventorsconcluded that the manufacturer was to select any cleaning methodappropriate to the predicted contaminants.

Accordingly, the fabrication process is modified as shown in FIG. 9. Inthis instance, the analyst applies plural critical values to theabsolute values of the differences. In this instance, the absolutevalues are compared with two critical values for two kinds of cleaningmethod. If the absolute values are closer to the first critical valuethan the second critical value, the manufacturer proceeds to step S15A.On the other hand, if the absolute values are closer to the secondcritical value than the first critical value, the manufacturer proceedsto step S15B. The first kind of cleaning method is, by way of example,different from the second kind of cleaning method in cleaner usedtherein, the concentration of the cleaner, cleaning temperature and timeperiod to be consumed.

The accumulated current value is assumed to be fallen within the rangebetween 2.0 pA and 2.5 pA under the conditions that the accelerationenergy and the beam current are 500 eV and 30 pA. The manufacturerproceeds to the first cleaning S15A, and the silicon wafer isdecontaminated in cleaning solution containing hydrofluoric acid andwater at 1:100 at 22 degrees in centigrade for one minute. On the otherhand, when the accumulated current value is fallen within the rangeequal to or greater than 1.8 pA and less than 2.0 pA, the silicon waferis decontaminated in cleaning solution containing hydrofluoric acid HF,hydrogen peroxide H₂O₂ and water H₂O at 1:1:100 at 22 degrees incentigrade for 0.5 minute.

As will be appreciated from the foregoing description, the surfacecontamination analysis is based on the current flowing out from a targetregion under the electron beam radiation. Even if the target region hasa complicated contour and is contaminated with plural contaminants, thedegree of contamination is accurately decided without damage to thetarget region. The incident angle of the electron beam is variable sothat the electron beam reaches the bottom of a micro-hole with a largeaspect ratio. Moreover, the electron beam current is selectable in thewide range, and is focused on an extremely narrow region. Thus, theanalyst precisely evaluates the surface contamination on a targetsubstrate.

When the surface contamination analysis is introduced into a process forfabricating semiconductor devices, the manufacturer evaluates thesemiconductor wafers at any point in the process sequence, andselectively decontaminates the seriously contaminated semiconductorwafers. Thus, the manufacturer prevents the fabrication system from theundesirable contaminants, and enhances the production yield and thereliability of products.

Although particular embodiments of the present invention have been shownand described, it will be apparent to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the present invention.

For example, the accumulated current values at certain points in thefabrication system may be stored in certain files, respectively. In thisinstance, the analyst watches the variation of the accumulated currentvalues at each point to see whether or not the fabrication apparatus atthe previous stage is troubled. If the accumulated current values at thecertain point, the analyst predicts the trouble for trouble shooting.Even tough there are plural steps until the certain point, the analystspecifies the apparatus in trouble on the basis of the characteristicpatterns of the accumulated current values.

1. A method for investigating a degree of contamination on a target region of a wafer, the method comprising: irradiating the target region with an electron beam; measuring the amount of current flowing out from the target region in response to the irradiating the target region with the electron beam; comparing the amount of current flowing out from the target region in response to the radiation of the electron beam with the amount of a current flowing out from a reference region to determine a difference there between; and detecting contaminants on the target region on the basis of the difference.
 2. The method of claim 1 wherein measuring the amount of current flowing out from the target region in response to irradiating the target region with the electron beam includes: irradiating the target region with the electron beam at a first incident angle; measuring the amount of current flowing out from the target region in response to the irradiating the target region with the electron beam at the first incident angle; irradiating the target region with the electron beam at a second incident angle; and measuring the amount of current flowing out from the target region in response to the radiation of the electron beam at the second incident angle.
 3. The method of claim 2 further including determining the degree of contamination on the target region on the basis of the difference between the amount of current flowing out from the target region in response to the radiation of the electron beam at the first incident angle and the amount of a current flowing out from the target region in response to the radiation of the electron beam at the second incident angle.
 4. The method of claim 2 further including generating a three-dimensional representation of the degree of contamination using the amount of current flowing out from the target region in response to the radiation of the electron beam at the first and second incident angles.
 5. The method of claim 1 further including evaporating at least one of the contaminants from the target region after detecting the at least one of the contaminants on the target region on the basis of the difference.
 6. The method of claim 1 further including heat treating the wafer to evaporate at least one of the contaminants from the target region.
 7. The method of claim 6 wherein, after heat treating the wafer, the method further includes: irradiating the target region with the electron beam; and measuring the amount of current flowing out from the target region, in response to the radiation of the electron beam, to detect contaminants on the target region.
 8. The method of claim 1 further including heat treating the wafer, in an inert atmosphere, to evaporate at least one of the contaminants from the target region.
 9. The method of claim 1 wherein measuring the amount of current flowing out from the target region in response to irradiating the target region with the electron beam includes: irradiating the target region with the electron beam at a first acceleration energy; measuring the amount of current flowing out from the target region in response to the irradiating the target region with the electron beam at the first acceleration energy; irradiating the target region with the electron beam at a second acceleration energy; measuring the amount of current flowing out from the target region in response to the radiation of the electron beam at the second acceleration energy; and detecting at least one contaminant an the target region using the amount of current flowing out from the target region in response to the radiation of the electron beam at the first acceleration energy and the amount of a current flowing out from the target region in response to the radiation of the electron beam at the second acceleration energy.
 10. The method of claim 9 further including generating a three-dimensional representation of the degree of contamination using the amount of current flowing out from the target region in response to irradiating the target region with the electron beam at the first and second acceleration energies.
 11. The method of claim 1 further including: measuring the amount of current flowing out from the target region, at a first temperature, in response to the irradiating the target region with the electron beam; and measuring the amount of current flowing out from the target region, at a second temperature, in response to the irradiating the target region with the electron beam.
 12. The method of claim 11 further including detecting a difference between the amount of current flowing out from the target region in response to irradiating the target area with the electron beam at the first temperature and the amount of current flowing out from the target region in response to irradiating the target area with the electron beam at the second temperature.
 13. The method of claim 12 further including generating a three-dimensional representation of the degree of contamination using the amount of current flowing out from the target region in response to the radiation of the electron beam at the first and second temperatures.
 14. A method for investigating contamination on a target region of a wafer, the method comprising: irradiating the target region with an electron beam; measuring the amount of current flowing out from the target region in response to the irradiating the target region with the electron beam; measuring the amount of current flowing out from a reference region in response to the irradiating the reference region with the electron beam; and detecting at least one contaminant on the target region using the amount of current flowing out from the target region in response to the radiation of the electron beam and the amount of a current flowing out from the reference region in response to the radiation of the electron beam.
 15. The method of claim 14 wherein measuring the amount of current flowing out from the target region in response to irradiating the target region with the electron beam includes: irradiating the target region with the electron beam at a first incident angle; measuring the amount of current flowing out from the target region in response to the irradiating the target region with the electron beam at the first incident angle; irradiating the target region with the electron beam at a second incident angle; measuring thin amount of current flowing out from the target region in response to the radiation of the electron beam at the second incident angle; and detecting at least one contaminant on the target region using the amount of current flowing out from the target region in response to the radiation of the electron beam at the first incident angle and the amount of a current flowing out from the target region in response to the radiation of the electron beam at the second incident angle.
 16. The method of claim 14 further including heat treating the wafer, in an inert atmosphere, to evaporate at least one of the contaminants from the target region.
 17. The method of claim 14 wherein measuring the amount of current flowing out from the target region in response to irradiating the target region with the electron beam includes: irradiating the target region with the electron beam at a first acceleration energy; measuring the amount of current flowing out from the target region in response to the irradiating the target region with the electron beam at the first acceleration energy; irradiating the target region with the electron beam at a second acceleration energy; measuring the amount of current flowing out from the target region in response to the radiation of the electron beam at the second acceleration energy; and detecting at least one contaminant on the target region using the amount of current flowing out from the target region in response to the radiation of the electron beam at the first acceleration energy and the amount of a current flowing out from the target region in response to the radiation of the electron beam at the second acceleration energy.
 18. The method of claim 17 further including generating three-dimensionally representation of the degree of contamination using the amount of current flowing out from the target region in response to irradiating the target region with the electron beam at the first and second acceleration energies.
 19. The method of claim 14 further including: measuring the amount of current flowing out from the target region, at a first temperature, in response to the irradiating the target region with the electron beam; and measuring the amount of current flowing out from the target region, at a second temperature, in response to the irradiating the target region with the electron beam.
 20. The method of claim 19 further including detecting a difference between the amount of current flowing out from the target region in response to irradiating the target area with the electron beam at the first temperature and the amount of current flowing out from the target region in response to irradiating the target area with the electron beam at the second temperature.
 21. The method of claim 20 further including generating three-dimensionally representation of the degree of contamination using the amount of current flowing out from the target region in response to the radiation of the electron beam at the first and second temperatures. 